The present invention relates to a semiconductor device, and more particularly to a power thyristor with a large critical rate of rise of on-state current (di/dt) and free from a finger voltage phenomenon.
It has been known to construct a thyristor with a steep rise characteristic of an ON current and a large critical rate of rise of on-state current (di/dt). Such a prior art thrysitor has a structure as shown in FIGS. 1 and 2, for example. The thyristor shown in those FIGS. is made of a first semiconductor layer 12 of N conductivity type, second and third semiconductor layers 14 and 16 of P conductivity type formed on both major surfaces of the first layer 12, a main emitter region 18 of N conductivity type and an auxiliary emitter region 20 of the same type, both being formed in the second layer 12, a cathode electrode 22 formed on the main emitter region 18, a gate electrode 24 formed on the second semiconductor layer 14, an anode electrode 26 formed on the third semiconductor layer 16, and an auxiliary electrode 28 formed on the second semiconductor layer 14, being disposed along an outer periphery of the cathode electrode 22. The cathode electrode 22 and the N region 18 cooperatively form an emitter section of a main thyristor element, while the auxiliary electrode 28 and the auxiliary emitter region 18 make up an emitter section of an auxiliary thyristor element. The thyristor with such a structure as shown in FIGS. 1 and 2 operates in such a way that a gate current 1 flowing through the gate electrode 24 first turns on the auxiliary thyristor, and a turn-on current 2 flows into the main thyristor element in the form of a trigger current 3 for the main thyristor element thereby turning on the main thyristor element. Thus, the prior art thyristor makes use of a called amplification gate function. Therefore, the trigger current 3 to the main thyristor element is large, so that an initial turn-on region of the main thyristor element is wide. This provides an abrupt rise of the ON current and a large critical rate of rise of on-state current (di/dt).
In the prior thyristor as shown in FIGS. 1 and 2, the turn-on of the main thyristor element is performed through two steps as described above. In the course of the two-step turn-on process, a finger voltage V.sub.fin phenomenon takes place as shown in FIG. 3. The finger voltage phenomenon offers no problem in the use of a single thyristor or when two or more parallel-connected thyristors are turned on with a high voltage applied to between the anode and cathode. However, the finger voltage phenomenon is problematic when two or more parallel-connected thyristors are turned on when a low voltage is applied therebetween.
To further discuss the finger voltage phenomenon, a simple model circuit will be used in which two thyristors SCR.sub.1 and SCR.sub.2 each having the structure as shown in FIGS. 1 and 2 are inserted between terminals P and Q, being respectively connected at the cathodes to associative resistors, and at the anodes together to an inductance L.
Assume that, in the model, a total current of 6,000 A flows through a P-Q path, and that the current equally shunts into the two thyristors SCR.sub.1 and SCR.sub.2. On this assumption, 3,000 A current flows into each of the thyristors. Even if trigger currents are concurrently fed to the two thyristors SCR.sub.1 and SCR.sub.2, only one of the thyristors is turned on when there is non-coincidence between the finger voltages of the thyristors SCR.sub.1 and SCR.sub.2, so long as the resistances of the resistors R.sub.1 and R.sub.2 are properly selected allowing for the finger voltages. Assuming that the 3,000 A current flows into the thyristor SCR.sub.1, following the turn-on of the thyristor SCR.sub.1, a voltage drop V.sub.AB as the sum of a voltage drop across the thyristor SCR.sub.1 at the time of the 3,000 A current passage through the thyristor SCR.sub.1 and a voltage drop across the resistor R.sub.1, appears between junction points A and B.
The thyristor SCR.sub.1 has a voltage vs. current characteristic as shown in FIG. 3. The voltage drop across the thyristor SCR.sub.1 is 1.5 V when the 3,000 A current flows therethrough. The resistance of the resistor R.sub.1 is 0.2 m.OMEGA.. The voltage drop V.sub.AB is EQU V.sub.AB =1.5 V+(3,000.times.2.times.10.sup.-4)=2.1 V.
Accordingly, to turn on the thyristor SCR.sub.2, its finger voltage V.sub.fin must be below a difference when the voltage drop across the resistor R.sub.2 at the time of the 3,000 A current passage is subtracted from the 2.1 V. The adjustment of the resistances of those resistors R.sub.1 and R.sub.2 is not only cumbersome, time-consuming, and superfluous but also poor in the accuracy. An approach to use a large resistor for the resistor R.sub.1 enables the thyristor SCR.sub.2 to turn on reliably even though the finger voltage is relatively large. The approach, however, involves a problem of heat dissipation because of the large heat generated. The turn-on of two or more thyristors in a well-balanced state necessitates that the finger voltage be as small as possible. Nevertheless, the conventional thyristors have been unsuccessful in reducing the finger voltage V.sub.fin to a desired degree or a negligible one.